Typical reduction systems in which the present invention may be usefully employed are disclosed in U.S. Pat. Nos. 3,765,872; 3,748,120; 4,528,030; and 4,834,792. Several methods have been used in the past for measuring and controlling the reduction rate in such systems as those disclosed in U.S. Pat. Nos. 3,601,381; 4,121,922; 4,153,450 and Japanese Patent Application No. 54-22226 dated Feb. 26, 1979. The content of these patents, particularly 4,153,450 are incorporated herein by reference.
U.S. Pat. No. 3,601,381 suggests a pair of gas sampling probes, are positioned in the (H.sub.2 & CO)-containing reducing gas inlet to a sponge iron reduction reactor and the other positioned laterally within the reactor at or slightly below said inlet, which probes connect to respective gas analyzers for comparing CO.sub.2 content at both probe positions. Assertedly, if the CO.sub.2 content is essentially equally low, then the sponge iron which is positioned ready for discharge at that point is sufficiently reduced; while a higher CO.sub.2 interior reading (relative to the external inlet reading) would indicate insufficient reduction and require interruption of the sponge iron discharge while reduction continues on the slowed or immobilized burden.
Although the invention of U.S. Pat. No. 3,601,381 relates to a commercialized process, to applicant's knowledge it has never itself been commercially used. This is not surprising since the interior gas sampling probe is not practical either as a permanent installation (being in too hostile an environment) nor as an intermittent probe (being too obstructed) and because of the unstable conditions present at the indicated interior point of sampling.
In U.S. Pat. No. 4,121,922 a batchwise gaseous reduction of metal oxide ores in a multi-stage gaseous reduction system is shown in which separate bodies of metal bearing material are simultaneously treated in a plurality of reactors to produce sponge metal, wherein the flow and composition of the reactor feed gas is determined before it is heated and fed to the reactor, the flow and composition of the reactor effluent gas is determined after it has been cooled and de-watered and the measured values of these flows and compositions are correlated to determine the rate of oxygen removal from the bed of metal bearing material in the reactor. The rate of oxygen removal then can be integrated to provide a signal continuously indicative of the percentage reduction of the reactor. Thus, as shown in the preferred example, it is possible by using a hydrogen balance and an oxygen balance to obtain an acceptable determination of the oxygen removal from the iron ore in the reduction reactor without making any measurement of the change in the amount of water vapor in the gas. Since this method is based on material balances, the substraction of big molar flows with little difference in magnitude between them considerably amplifies the measurement errors.
As can be seen from the foregoing, this method lacks simplicity and because of its mathematical integrations and sampling procedures and the special material balance involved its application is difficult.
In U.S. Pat. No. 4,153,450 molar flow rates and compositions are measured in spent and reducing gases leaving from and being fed to a moving bed vertical reduction reactor and are correlated with the reducible oxygen content of the ore and with the sponge iron production rate to determine an estimated percentage of reduction.
During passage of the reducing gas through the reduction reactor the gas composition changes as an incident of the removal of oxygen from the iron ore and hence it is theoretically possible to determine indirectly the percentage of reduction by measuring changes in the composition of the gas flowing through the reactor. However, numerous practical problems are encountered when an effort is made to measure the percentage of reduction. In the first place there is an elapsed time of several hours between the instant when the constituents of the spent gases are measured and compared with the reduction gases being fed to the reduction reactor, and the instant when the burden being reduced is discharged from the reactor and its real reduction rate is actually measured. Furthermore, as said measurements involve a special and very precise material balance between said compositions at the inlet and at the outlet of the reactor, such process is difficult and impractical. Thus, even though changes can be made to modify the reduction rate, there is an uncertainty on the effect that such changes will produce and the effect can only actually be measured some time later.
A further problem with this method arises with respect to choosing the point of measurement. Any point outside a reactor has the disadvantages discussed above. Thus, the preferable point of measurement would seem to be internal to the reactor. However, the latter also has inherent problems which arise from the fact that the reduction reactor is a pressure vessel with a hostile high temperature environment having closely packed particles with gas flows whose composition can be very uneven, being particularly biased at the points of injection and removal (caused by uneven flow patterns and a chemical instability resulting from fluctuations in the pressure of the gases due to the moving burden). Even direct analysis of samples of partially-reduced ore particle specimens from within the reactor is known not to be reliable, because the degree of metallization of any such sample can vary significantly from the average overall degree of metallization of the burden as a whole because of the unevenness of particle size, of gas and solid flows, of porosity, and the like. See for example the comments in the Japanese reference discussed next.
Japanese Patent Application No. 54-22226 shows an example in a fixed bed or batchwise iron oxide reduction process, wherein the degree of metallization is derived from a previously-determined relationship between reduction speed and degree of metallization for specific operating conditions and a given raw material. The Japanese applicant noted that as the reduction reaction progresses the reduction speed decreases as the degree of metallization increases, and such decrease results in a decrease in the difference in the amount of oxygen contained in the gas between the inlet and the outlet of the furnace. However, such change in the amount of oxygen in the composition of the gas may be unreliable, because the oxygen measured is involved in several successively occurring reactions.
It was also stated that when the flow rate and composition of the gas are maintained substantially constant at the inlet of the furnace, then their values need be measured only once and such values may be used in succeeding calculations and thereafter the gas analysis need be performed only at the outlet of the furnace. It was suggested that this method can be applied to a moving bed reduction reactor by establishing different locations for measurement both inside and outside the reactor, but apart from general considerations such as reducing time and speed of the moving material, this reference does not teach any specific or practical way in which this can actually be accomplished.
When the reduction speed is decreased, the metallization increases, so that the differences between two measuring points are also decreased to a very small value. Thus, the Japanese applicant recommend adopting various procedures in combination in an effort to make their method useful. From the foregoing, it is apparent that this method is difficult to implement due to the fact that a change in metallization requires the implementation of numerous changes in the operating variables which are not desirable in a moving bed reduction process because of the operational problems that result which are worse than the problems solved. It is also noted that this method is time dependant. Thus it is necessary to compensate for the elapsed time between the measurements at the measuring points and furthermore to take into consideration the analyzing time.
In addition to the foregoing problems, often the operating parameters may have to be changed to accommodate the type of ore or to modify the characteristics of the desired final product of the direct reduction. This is true, for example, when a lesser degree of reduction is not only permissible but desired; such as when a further processing step does not require or even is adversely affected by a higher degree of reduction. Modern pre-reduction and smelting and refining processes of iron ores for steelmaking are negatively affected by too high a degree of pre-reduction. Thus, from the foregoing it will be apparent that the prior art methods of process control are complicated and still lack the precision and speed necessary to give dependable and usefully timely values for the reduction rate and/or for the degree of metallization.